Recently (as in the last three Sundays..laundry day) when the washer drains the first time there's a sewer gas odor in the basement. The odor goes away relatively quickly and does not return through 3-4 more loads of laundry. The toilet has also started bubbling as the washer drains and if you flush the toilet it is VERY slow to drain. I think its a bubble vs a gurgle, I'm not sure how to tell the difference.

The sink and tub in that bathroom seem to drain just fine, though I haven't run an excess of water down them while this is occurring (just turned the water on for a minute or so).

The toilet remains slow for 12ish hours, meaning I've gone back down a few hours after laundry and it's still slow, but the next morning it's always fine.

I haven't altered anything and like I said it just started happening recently.

Because the WM dumps lots of water quickly because it has a pump. Once the pipe gets full from the partical clog back to where the toilet is, the toilet can't dump its water and waste into the mostly full pipe. If it can't move fast enough, it won't empty. Higher stuff from above can partially fill the pipe.